Autotune Mobile Antenna

ABSTRACT

An automatically tunable mobile antenna is provided with toroidal inductors connected in series between the antenna feed point and a whip and a shunt inductor to ground at the RF input, with the inductors forming an L network impedance matching circuit having values which are in a binary sequence and which are selectively added to impedance match the whip to the output impedance of a transmitter.

FIELD OF INVENTION

This invention relates to mobile antennas and more particularly to anautomatically tunable mobile antenna having inductors connected inseries at the base of a whip having values which are in a binarysequence and which are selectively shorted or un-shorted to impedancematch the whip to the output impedance of a transmitter.

BACKGROUND OF THE INVENTION

Mobile high frequency or HF, 3-30 mHz, antennas have in general beenshort for the frequency of operation. Because they are short, theantennas loading coils are used to cancel out the capacitive reactanceassociated with short antennas, normally whip antennas. The loading coilcan be placed at the base of a whip or can be put in the center of thewhip which is usually somewhere between 6 and 10 feet long. One can alsoput the loading coil at the top of the whip with a capacitive top load.While this top loaded configuration works, the antenna be can be made tooperate effectively by bottom loading the whip because it takes lessinductance.

In the past mobile antennas have been singled banded, meaning theyoperate in one frequency range. These antennas can be made multi-bandedby changing the frequency and tapping the coil used to load the antennaand shorting out the remainder of the coil. In a fairly recentinnovation in the past 20 years so-called screwdriver antennas have beendeveloped which are basically a center loaded antenna having a variableturn coil. It will be noted that in these configurations the coil is afairly large in both length and diameter and usually has a cover thatgoes from the bottom of the coil to the top. The cover and internalshorting circuitry is motor driven to move to short out portions of thecoil as it is extended or contracted such that portions of the coil areshorted except the part of the coil that is used for the matching.

Typically in such coils the movable tap is driven by a DC motor with themotor being stopped at the point when the standing wave ratio or SWR isat a minimum. However in order to change bands with such an antenna, theamount of time utilized in driving the motor is excessive such that togo from one band to another may take as many as 3 minutes. This isinconvenient when one wishes to shift from one band to another. It islikewise inconvenient when within a band one significantly tunes off thefrequency at which the coil was originally set. Moreover in the past fornon-automatic screwdriver antennas, the coil is set by hand which forinstance requires the driver to get out of the car and move the tap.

In order to solve the inconvenience described above, there have beenattempts to locate an antenna tuner at the base of the whip toeffectuate impedance matching. However antenna tuners are far lessefficient than the use of a loading coil because of the straycapacitance at the output of the tuner. The capacitance and radiationresistance of the antenna is what is being fed by RF energy. This straycapacitance is in parallel with the capacitance associated with theantenna itself. Thus, when one applies RF current to the antenna, thecurrent is divided between the antenna and the stray capacitance. Notethat the current created in the antenna causes radio waves to radiate.The more current that one can get into the antenna whip the more it willradiate and the better it will perform. However if more current is goinginto stray capacitance then the amount of radiated power is diminished.While the tuner itself may include loading coils, it is nonethelessimportant to minimize stray capacitance by locating the loading coils onthe antenna whip itself where the loading coil is not touching anythingexcept the whip. This minimizes stray capacitance provides a far betterpower transfer to the antenna. In antenna tuners, any loading coils arelocated within the antenna tuner itself.

Thus the use of antenna tuners at the base of a whip has been largelybeen rejected and automatic screwdriver antennas have been substitutedfor the use of these antenna tuners. However these automatic screwdriverantenna tuners are expensive and require either a manual or an expensivecontroller. Due to the external coil and the tapping arrangement theseantennas are big and heavy and are extremely costly. Moreover they areunsightly if one is attempting to get a big efficient antenna. The smallones are better looking but do not work as well because of the Q of thecoil. It is noted that one can hardly obtain an unloaded Q better than500 to 600 out of any free standing coil and this requires relativelylarge size coils. Moreover large coils with such a high Q limits theeffective usable bandwidth of the antenna once it is tuned. Thus thereis requirement for efficient mobile antennas to provide a high Q coilwithout being unsightly, large and expensive.

There is however a base-loaded tunable mobile antenna produced by theBarrett Corporation of Australia which utilizes a series of air woundloading coils in a housing which are connected together form theimpedance matching function. The system requires a specializedtransformer between the lower of the coils and the antenna feed point totransform the antenna impedance into one that matches the output of atransceiver, usually around 50 ohms. However it is only with difficultythat these antennas can be made to match the transceiver outputimpedance. It is noted that when the impedance matching offered exceedsa 2:1 SWR ratio there is a folding back of the transmit power so thatthe antenna presents an SWR less than 2:1 SWR to the particular radio towhich is coupled. This requires specialized transformers that aredesigned for a particular transceiver. However, in terms of generalpurpose amateur radios, absent a perfect match, these radios fold backthe power so that these antennas do not always work particularly well.

Moreover, the Barrett antenna utilizes air wound coils which when placedin proximity to each other crosstalk each other such that the ability toeffectuate a perfect match between the whip and the transceiver isimpacted at various frequencies, making the matching unstable. In aneffort to reduce crosstalk, the air wound coils are oriented at rightangles to each other. However this technique only marginally reducescrosstalk.

Furthermore, if the relationship of the inductance values of each of thecoils is not binary related, which makes switching schemes to switchthese coils in and out is designed as an ad hoc process.

Finally in the Barrett antennas, switching software is located at thebase of the antenna where RF fields are high and oftentimes interferewith the semiconductor switching circuits located at the base of thewhip. Housing the electronics for switching the coils of the Barrettantenna at the base of the whip thus presents instability problems,especially for the high currents involved when driving a whip likeantenna.

There is therefore a need for an automatic antenna tuning system formobile whip antennas to eliminate the aforementioned problems.

SUMMARY OF THE INVENTION

In the subject invention a number of series connected toroidal coils areconnected between the antenna feed point and a whip, with the inductancevalues of the toroidal coils being in a binary sequence such that forinstance the inductance values of the coils might be for instance 2micro-henrys, 4 micro-henrys, 8 micro-henrys, 16 micro-henrys etc. Notethat toroidal inductors are utilized due to the fact that the RF energyis contained within the toroid itself. Relays are placed across thevarious toroidal coils to unshort the coils in accordance with theoutput of a controller which is located remote from the antenna andusually at the transceiver located within a vehicle. The granularity ofthe inductance values is determined by the coil having the leastinductance. Moreover, a shunt coil is located between the antenna feedpoint and ground to effectuate impedance matching to the normal 50 ohmoutput of a transceiver.

In one embodiment, a fiberglass rod is located within a generallycylindrical housing which supports a whip connector at the top of thehousing and runs down to the bottom of the housing at which point a ⅜×24threaded stud connector is located. The switching circuit is locatedwithin a housing that mounts normally closed relays. These relays aremounted on a circuit board running vertically and is attached to thefiberglass rod within the housing. The control head in one embodimentincludes a rotary encoder switch connected to the relays to control theswitching state associated with the relays to unshort the associatedcoils until such time as a minimum SWR is indicated by a meter on thecontrol head or on the transceiver. When a minimum SWR is achieved for agiven frequency, this is memorized by circuits within the control headsuch that in returning to the frequency, the particular relays whichoptimize the SWR for the frequency are opened. In an alternativeembodiment, the frequency from the transceiver utilized to drive themobile antenna is detected and the relays are set in accordance with thepreviously memorized settings.

It will be appreciated that because of the use of a binary sequence, theinductance steps are linear and additive, such that for each incrementin inductance, the next higher inductance is added to the lowerinductance. This is accomplished with the unshorting of the coils suchthat when each of the coils is unshorted, the added inductance iscumulative, with the amount of inductance presented between the antennafeed point and the whip increasing in a linear stepped manner.

In the tuning procedure, the amount of inductance switched in starts atthe lowest inductance and increases with the opening up of more relays.Thus in one embodiment the microprocessor controlled relays sequencesfrom a low inductance to high inductance such that more and moreinductance comes out of the bottom section of the unit in the samemanner as a screwdriver antenna uncovers increasing numbers of coils toadd inductance for antenna tuning.

In summary, an automatically tunable mobile antenna is provided withtoroidal inductors connected in series between the antenna feed pointand a whip. The fixed shunt inductor along with the series inductorsform an L network impedance matching circuit having values which are ina binary sequence and which are selectively added to impedance match thewhip to the output impedance of a transmitter.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the subject invention will be betterunderstood in connection with the Detailed Description in conjunctionwith the Drawings of which:

FIG. 1 is a diagrammatic illustration of the utilization of an automaticscrewdriver antenna attached to the bumper of a vehicle, illustratingthat tuning of the antenna requires driving of the inner coil of thescrewdriver antenna up and down until a suitable standing wave ratio isachieved;

FIG. 2 is a diagrammatic and cut away illustration of an automaticallytunable mobile antenna mounted to the rear bumper of a vehicle in whichbinary series related inductors are series connected and located withinan inductor housing having an RF input at the bottom and a whip mountedon top of the inductor housing, with control of the switching of theinductors in and out of the circuit controlled by a control headinternal to the body of the vehicle;

FIG. 3 is a schematic diagram of series-connected inductors connectedbetween the antenna input and the RF output connected to a whip antennawhich are selectively left open or shorted utilizing relays toselectively short selected coils for reducing the standing wave ratio toan acceptable level, with relay selection being under the control of amicroprocessor coupled to a rotary encoder switch to initially set therelays until an appropriate SWR match is achieved, also showing thatupon achieving of a suitable SWR for a given frequency or frequencyband, the relay states are memorized along with the frequency orfrequency band, with the relay states recallable when the operator oftransmitter wishes to operate on the memorized frequency or frequencyband;

FIG. 4 is a diagrammatic illustration of the control head mounted withinthe vehicle for use in initially setting relay states to achieve aminimum SWR and for thereafter recalling the relay states for a givenfrequency or frequency band;

FIG. 5 is a diagrammatic illustration and top view of the mounting ofeight inductors on a circuit board, the majority being toroidal tominimize cross talk, mounted to a central nonconductive support shaftwithin the inductor housing, also showing the location of relays tocontrol the amount of inductance added between the RF input to theantenna and the whip; and,

FIG. 6 is a detailed schematic diagram showing these switching circuitsutilized in the control of the relays illustrated in FIGS. 3 and 5.

DETAILED DESCRIPTION

Referring to FIG. 1, a vehicle 10 is provided with an automaticscrewdriver antenna 12 which is composed of a stationary cylindricalhousing 14 over which is mounted a translatable outer housing 16.Interior to housing 16 is a loading coil 17 attached at its top tohousing 16 and to one end to a whip 18. Note coil 17 translates asillustrated by double ended arrow 19 within stationary cylindricalhousing 14. Within cylindrical housing 14 is a shorting contact assemblywhich contacts coil 17 as it translates to short out various segments ofthe coil to establish tuning. Thus, the loading coil has turns which areshorted in accordance with the position of coil 17 and the shortingcontact in stationary housing 14, such that when coil 17 translates asillustrated by double ended arrow 20 the amount of interior coil shortedis controlled.

For a given length of antenna and a given exterior configurationinvolving the vehicle itself, a motor (not shown) drives coil 17up-and-down until the standing wave ratio presented by the antenna tothe transceiver within the vehicle is minimized. While this type ofantenna works satisfactorily and is relatively efficient, it sometimestakes as long as 3 minutes to be able to move the coil 17 up-and-downuntil the appropriate tap is made to the internally carried coil. Thuschanging frequency, and more especially when changing frequency bands,it takes a fair amount of time to be able to tune the mobile antenna toa particular band and thence to a particular frequency within the band.

Moreover, the weight of such an antenna is excessive and because of itssize and wind resistance it is only mounted with difficulty on avehicle. Additionally, the cost of such an efficient antennaincorporates not only the cost of the coil and sliding mechanism as wellas its housings, it also includes the cost of a drive motor and drivecontrol circuitry as well as SWR monitoring. Importantly, automaticscrewdrivers are said to be unsightly and for those wishing anonymity itcan hardly be said that such antennas will be relatively unnoticeable.

Rather than mounting the automatic screwdriver assemblage depicted inFIG. 1 on a vehicle, the subject automatically tuned antenna includes acylindrical housing 30 in which is housed serially connected coils 32which when selected provide a selectable amount of inductance to be ableto tune the whip antenna 34 to the output impedance of a transceiver 36housed within vehicle 38. It will be appreciated that the physical sizeof housing 30 is considerably less than that associated with theautomatic screwdriver antenna FIG. 1, and is considerably less costlyand less unsightly. Moreover wind resistance is kept to a minimum andbecause of its lightweight, the entire package may be mounted notnecessarily on the vehicle bumper, but may be mounted anywhere on thevehicle such as with a window mount or a roof mount due to the fact thatthe entire package weighs less than 12 ounces in one embodiment.

Most importantly, toroidal inductors are used to minimize interferencewith other coils, with the binary sequence coils connected in series toeffectuate a perfect match for a given frequency band when, for instancethe relays that control the shorting of the coils are set when asufficiently low standing wave ratio exists. The switching of the relaysis almost instantaneous such that one can go from one frequency band toanother almost instantaneously once the states of the relays for theband have been established. Moreover, control for the relays comes froma control head 40 within the body of vehicle 38 which is removed fromthe high current and voltage conditions at the mobile antenna. Removalof the control circuitry from the antenna is important because in thepast RF fields from the antenna can affect electronic circuits locatedat an antenna. These RF fields can cause instability and because thecontrol head 40 is within the vehicle which functions as a Faraday cage,the stability of the tuning of the antenna is not deleterious theaffected by RF transmissions.

Also central to the stability of the mobile antenna is the use oftoroidal inductors where needed. It is a feature of toroidal inductorsthat the RF fields are located solely within the torus and thus there isno crosstalk between the toroidal inductors. As a result, there is nonecessity to calculate the interaction between inductors when designingthe inductor circuit. Furthermore, the values of the inductors arebinarily related such that if for instance the smallest inductor is 2microhenrys, the next larger inductor has a value of 4 microhenrys, withthe next larger inductor having a value of 8 microhenrys. In short, thevalues of the inductance is are multiplied by 2 for each step. Note alsothat the granularity of the tuning is determined by the inductor havingthe lowest inductance. Thus when all the inductances are added togetherto create an acceptable standing wave ratio, the combination ofinductances can be tailored in a cut and try operation to minimize thestanding wave ratio.

Additionally, a shunt coil is connected between the antenna feed pointand ground to match the impedance at the base of the antenna to theoutput impedance of the transmitter to which the antenna is connected.

Tuning of the subject antenna is quite easy. The easiest way to tune theantenna is to listen to a receiver coupled to the antenna and to turnthe rotary tuning knob until one obtains maximum noise. In oneembodiment a knob push of the tuning knob increases tuning speed, suchthat the speed with which the relays are changed increases by a factorof 10 when the knob depressed. Rotation of the knob results in adding orsubtracting inductance with each rotary click of the knob. After coursetuning is achieved, the knob is again pressed such that the tuningcontrol goes to a slow mode. This permits one to transmit and observethe SWR until fine tuning of the inductance to the whip results in a lowSWR.

Once a low SWR is achieved for a given band or a given frequency, in oneembodiment the relay states are set with the touch of a separate buttonand a light emitting diode or LED will blink telling the operator thatthe state of the relays that resulted in the low SWR is storedtemporarily in memory. Then a second knob is turned to the band orfrequency to be permanently stored with the related relay states. Whenthe aforementioned LED goes out the information is transferred fromtemporary memory to permanent memory at the band position indicated bythe second knob. Thereafter if one wishes to go to the particularfrequency or band one simply rotates the second knob to the positioncorresponding to that particular band or frequency and the relays willbe set in accordance with the previously memorized states.

As will be described, mobile antenna matching utilizing selectableseries connected inductors is facilitated in a small package which isboth lightweight and inexpensive and which is mountable anywhere on avehicle with a minimum amount of specialized mounting hardware. In oneembodiment the connector at the base of the inductor housing is a commonthreaded stud utilized in mounting a large variety of antennas to mobilemounts.

Referring now to FIG. 3, in one embodiment a number of series connectedinductors 42, 44, 46, 48, 50, 52, 54 and 56 are serially connectedbetween an input antenna terminal 58 and an output terminal 60 to amobile antenna, normally a whip is mounted. The whip presents animpedance which is generally quite high due to its short length formobile applications and the series connected loading coils areinterposed between the antenna feed point and the base of the whip tobring down the associated impedance to that which matches the outputimpedance of the transmitter to which the antenna is coupled. In oneembodiment a series of relays, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80and 82 are shown initially shorting out the associated coils. In orderto match the impedance of the shortened antenna to the transmitteroutput impedance a microprocessor 84 controls the position of theswitches to short associated coils. In one embodiment antenna matchingis accomplished on a cut and try ad hoc basis utilizing a digital rotaryencoder switch 86, decoded by microprocessor 84 which selectively shortsvarious of the associated coils. During this process the standing waveratio is measured by SWR meter 88 to indicate when in the rotation ofthe rotary encoder switch a minimum SWR has been achieved. When aminimum SWR has been achieved, memory within microprocessor 84 is set asillustrated at 90 which memorizes the relay states as well as thefrequency band 92 of the RF driving the antenna.

Having memorized the frequency or frequency band and the switch statesone can return the relays to the required states for the desiredfrequency or frequency band.

It is noted that a shunt coil 94 is utilized to match the tuned antennato the transmitter output impedance as is common with screwdriverantennas. Note that there is no specialized impedance transformer at thebase of this antenna, with the inductors and the shunt coil providingall of the necessary inductance values for the matching.

In this embodiment, there are a number of toroids which are controlledover a multiline cable 96 connected by connector 98 to microprocessor84. As illustrated, these lines connect to relay actuators 100, 102,104, 106, 108, 110, 112, 114, 116, 118, and 120. These drive circuitsare isolated from any RF fields due to the capacitor and diode networkscoupled to the input to these actuators as illustrated by capacitor 122,diode 124, capacitor 126 for actuator 100 and additionally diode 128 andcapacitor 130 for actuator 102. While in some cases it is only necessaryto provide one relay to short an inductor, in some instances thevoltages at the inductor are relatively large, necessitating seriesconnected relays. Relays of a common variety can handle 1000 volts.However when serially connected they can handle twice the voltage. Thusit is not necessary in the high RF environment of the mobile antenna toutilize exotic reed switches which are both bulky and expensive, butrather one can utilize standard inexpensive relays, connected in seriesto be able to withstand the high voltages at various points in thecircuit. Here the double relay configuration is utilized to shorttoroidal inductor 52, toroidal inductor 54, and toroidal inductor 56.

Referring now to FIG. 4, when the mobile antenna of FIG. 2 is driven bya transceiver 130, for instance set to 14.300 MHz, control head 40 inone embodiment includes rotary encoder switch 86 which is utilized toset the switch states of the relays FIG. 3. A button 132 is utilized tomemorize the switch states of the relays when the standing rave ratioset by the relays is at an acceptable level. This may be determined byan SWR meter on the transceiver itself or by a separate SWR meter on thecontrol head. As mentioned before, the frequency or frequency band ismemorized at microprocessor 84. When wishing to return to a givenfrequency or frequency band a rotary frequency band switch 86 isutilized to read out the memory corresponding to the frequency band andreturn the relays to the switch states associated with the frequency inthe frequency band. The frequency band selected by switch 86 may beindicated on meter 134 so as to give the radio operator an indication ofthe frequency band to which the antenna is being tuned. Further, rotaryswitch 86 may be replaced by multiple push button switches.

Referring now to FIG. 5 the mechanical construction of housing 30 isillustrated, here a cylindrical housing 30 houses a central verticallyupstanding nonconductive rod 142 which is attached to a threaded stud(not shown) and a top whip connector (not shown). Affixed to thiscentral rod is a circuit board 142 on which are mounted coils 42, 44,46, 48, 50, 52, 54 and 56. The shorted or open state of each of thesecoils is controlled by relays 62, 64, 66, 68, 70, 72, 74, 76, 78, 80 and82.

In operation, relay 62 is connected to short coil 42 utilizing circuitboard traces 63 and 65, whereas relay 64 shorts out coil 44 utilizingtraces 65 and 67. Relay 66 shorts out coil 46 utilizing traces 67 and69, and relay 68 shorts out coil 48 utilizing traces 69 and 71. Relay 70shorts out coil 50 utilizing trace 71 and trace 73. Relays 72 and 74 areserially connected together such that in series they short coil 52utilizing traces 73 and 75. Relays 76 and 78 are serially connectedtogether and short out coil 54 utilizing traces 75 and 77, whereasrelays 80 and 82 are serially connected together and are used to shortout coil 56 utilizing traces 77 and 79. The relays can be any relayscapable of handling the required voltage and current and can be singlerelays not in series.

It will be noted that coils 46 and 50 are preferably air wound, whereasthe remainder of the coils are toroidal coils used to preventinterference and crosstalk. The air wound coils are located sufficientlyfar apart to eliminate crosstalk and are used because for their lowinductance values, air wound coils are much more efficient. However themajority of the coils are toroidal coils, used to eliminate crosstalk,keep the coil sizes small and increase the stability of antennaoperation. Also, mounted outside the inductor housing is shunt coil 94as illustrated. It will be appreciated that all of the coils, bothtoroidal or air wound, as well as the relays are housed within housing30 and are mounted to the aforementioned central shaft.

In one embodiment, the inductances of the 8 coils are given by thefollowing table:

TABLE I L1 0.070 μH L2 0.140 μH L3 0.281 μH L4 0.562 μH L5 1.125 μH L6 2.25 μH L7  4.5 μH L8    9 μH

Note that the order of the mounting of the coils on the circuit boarddoes not necessarily reflect the binary series of inductance values andtheir location is dictated by non-interference considerations andmechanical mounting convenience.

Referring to FIG. 6, a detailed schematic diagram illustrates thetransistorized switching circuits utilized to control of theaforementioned relays, as well as a mechanical rotary switch utilized toinitially manually set the relays. Also shown is a meter utilized inindicating the frequency band to which the autotune antenna is set.

In this figure, microprocessor 84 is utilized to actuate relay drivecircuits 200, each of which are composed of a sense transistor 202connected to the base of a high power switching transistor 204 such thatupon application of a drive signal over line 208 to the base oftransistor 202, current through this transistor brings down the voltageat the base of transistor 204 to turn transistor 204 on. The emitter oftransistor 204 is connected to the B+, in one embodiment 12 V, such thatwhen transistor 204 is turned on, this voltage is applied from thecollector of transistor 204 to the associated relay drive as illustratedat 208. Note that a capacitor 210 runs from B+ to ground, whereas acapacitor 212 runs from the collector of transistor 204 to ground forfiltering out stray RF.

It will be noted that pin 14 of microprocessor 84 provides a voltage tothe base of transistor 202, with pins 11 and 12 controlling the bases ofthe transistors corresponding to relays K2 and K−3. Control for thebases of transistors labeled K4-K9 are available from output pins 24-28of microprocessor 84 to control the associated relays

It will be appreciated that microprocessor 84 is utilized to actuate therelays associated with inductors L1-L8 under the control of a rotaryswitch generally indicated at 220. With each rotation of the switch forinstance clockwise, switch 222 is closed and microprocessor 84 isutilized to sequentially actuate the associated relays in an updirection, whereas when switch 220 is rotated counterclockwise, switch224 is closed, the relays are actuated in the down direction. Thedirection which the microprocessor is instructed to go in the sequencingof the relay states is dependent upon the clockwise or counterclockwiserotation of the rotary digital encoder switch. The speed by which themicroprocessor moves upwardly or downwardly through the relay states canbe increased by the closing of switch 226 such that when the switch isclosed as for instance by the depression of a button on the front panelof the controller, the relay states are rapidly cycled, whereas when theswitch 226 is not depressed, the relay states are changed in arelatively slow fashion.

As mentioned before when the standing wave ratio is indicated as beingwithin an acceptable range, the relay states are stored in themicroprocessor in accordance with the memory set by a second rotaryswitch 240 which establishes the band of interest. With the depressionof a switch here illustrated at 230, the switch states of the relays forthe selected band of interest is memorized, with the depression ofswitch 230 resulting in a signal being applied to input pin 3 ofmicroprocessor 84 to save the particular relay states in the designatedband memory when switch 230 is closed.

In one embodiment, the band of the saved relay states is indicated byanalog meter 234 so that the particular band being tuned is readilyobservable by the radio operator. Additionally an LED 236 is actuatedwhen the save button is pressed which is activated by a signal atterminal 15 to indicate that a particular relay state has been saved ina designated band.

In operation, the frequency band associated with the rotary encoder bandswitch is decoded by the associated switch position of switch 240 whichtaps a particular voltage from a resistor string composed of resistors242, 244, 246, 248 and 250, with the resistors having the resistancevalues illustrated. These resistor values correspond to 6 memorylocations corresponding to 6 bands. This type of rotary band encoderdecoding system requires only one lead from switch 240 to themicroprocessor, with the voltage on the lead determining which band isbeing tuned. Thus, the rotary switch band encoder positions areconverted into voltages to define a frequency band that relates tocorresponding relay states. While there only 6 positions illustrated,the number can be doubled so as to accommodate additional memorylocations corresponding to more frequency bands.

Having selected the particular band for which the antenna is to betuned, rotation of rotary encoder switch 220 provides for changing ofrelay states until such time as a suitable standing wave ratio isachieved. When this standing wave ratio has been achieved, pressing ofswitch 230 results in the saving of the relay switch states into theband designated by rotary encoder switch 240.

It will be appreciated that with 8 possibilities for the switchingstates associated with the operation of rotary encoder 220, the amountof inductance inserted between the antenna feed point and the antennawhip has 28 or 256 possible values, with the smallest increment beingthat associated with the smallest value of inductance for a coil, inthis case 0.070 μH. This gives a sufficient inductance range for a widevariety of operating conditions for whips for instance between 5 and 10feet in length, with the fine tuning granularity being provided by thecoil having the least inductance. When more inductance may be required,for instance for extending the operation from 40 m to 80 m, additionalcoils may be added in series.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications or additionsmay be made to the described embodiment for performing the same functionof the present invention without deviating therefrom. Therefore, thepresent invention should not be limited to any single embodiment, butrather construed in breadth and scope in accordance with the recitationof the appended Claims.

What is claimed is:
 1. An automatically tunable multiband antenna,comprising: an input feed point and a whip; a number of series connectedinductors, at least some inductors being toroidal and coupled betweensaid feed point to said antenna and said whip; a number of relays forselectively shorting associated inductors; and, a control head removedfrom the vicinity of said inductors for controlling the operation ofsaid relays.
 2. The antenna of claim 1, wherein said relays areconfigured to be normally closed to short the associated inductors. 3.The antenna of claim 2, wherein said relays are selectively un-shortedto insert inductance between said antenna feed point and said whip forthe purpose of minimizing the standing wave ratio of said antenna. 4.The antenna of claim 1, wherein the values of inductance of saidinductors are configured in a binary series.
 5. The antenna of claim 1,and further including a circuit board for mounting said inductors, saidinductors being mounted such that any non-toroidal inductors haveminimal effect on the other of said inductors.
 6. The antenna of claim5, and further including a housing for said inductors, said housingextending from said antenna feed point to said whip.
 7. The antenna ofclaim 6, wherein said housing includes a centrally mounted shaft runningfrom said antenna feed point to the point at which said whip is attachedto said housing.
 8. The antenna of claim 7, wherein said circuit boardis mounted to said centrally mounted shaft.
 9. The antenna claim 8,wherein said centrally mounted shaft includes a non-electricallyconductive material.
 10. The antenna of claim 1, wherein said antenna istuned to a frequency through the selective actuation of said relays andthe unshorting of the associated inductors.
 11. The antenna of claim 10,wherein during the turning of said antenna, RF energy is applied to saidantenna feed point and wherein said control head actuates said relaysuntil such time as said antenna exhibits a predetermined acceptable SWR.12. The antenna of claim 11, wherein said control head memorizes theswitch states of said relays when said predetermined acceptable SWR hasbeen achieved.
 13. The antenna of claim 12, wherein said control headmemorizes the frequency or frequency band of the RF energy applied tosaid antenna when said predetermined acceptable SWR has been achieved.14. The antenna of claim 13, and further including apparatus for settingthe frequency or frequency band to which said antenna is to operate byinterrogating the memory of said control head and for setting the switchstates of the relays in accordance with the switch states associatedwith said frequency or frequency band.
 15. An autotune mobile antennafor operation in a number of frequency bands, comprising: an L networkcomprising a number of binary series related series-connected inductorsinterposed between the feed point of said antenna and a whip, saidinductors being initially shorted by the actuation of relays coupledthere across, with said tuning accomplished through selective unshortingof said inductors along with a shunt inductor to ground at the RF inputend until such time during the application of RF energy to said antennafeed point, the SWR of said antenna achieves a predetermined SWR; and, acontrol head remote from said antenna for controlling the selectiveunshorting of said inductors.
 16. The antenna of claim 15, wherein atleast some of said inductors are toroidal inductors having fields whichare contained within the torus so as to minimally affect the operationof adjacent inductors.
 17. The antenna of claim 15, wherein the tuningof said antenna includes applying RF energy to the input to saidantenna, selectively un-shorting said inductors, measuring the SWRassociated with the antenna upon the application of said RF energy, andmemorizing the shorted or un-shorted states associated with theinductors when said predetermined SWR has been achieved.
 18. The antennaof claim 17, wherein for a predetermined frequency or frequency band,said control head sets the switch states of said relays to thatassociated with achieving said predetermined SWR.